Preheat oven to 300°F.
Spray large shallow roasting pan with cooking spray. Combine popcorn and nuts in pan; place in oven while preparing glaze.
Stir brown sugar, corn syrup, margarine and salt together in a heavy 2-litre saucepan. (Do not use a smaller pan, or mixture may overflow).
Stirring constantly, bring to a boil over medium heat. Without stirring, boil 5 minutes. Remove from heat; stir in vanilla and baking soda. Mixture will foam. Pour syrup mixture over warm popcorn mixture, stirring to coat well.
Bake, stirring occasionally, 45 minutes. Remove from oven. Cool; break apart. Store in tightly covered container.